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Ideal nightly discharge for off-grid battery bank?  RSS feed

 
Vlad Alba
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I'm monitoring my battery voltage throughout the day and night. (solar power)

What should it be getting up to, and for how long, during peak sunlight?
What's the ideal minimum for average nighttime low?

This is a 24 v flooded lead acid golf cart battery system. I'm thinking 28 volts during day for an hour or two and then at night the lowest the ideal would be no lower than 24.0, which would be a 50% discharge, I believe. Is that too low for it to be getting?

Of course, my charge controller manages how high and such during the day, but I also try to use power during the day rather than the night, so depending on what I use, that cuts into the absorption/float stages. I'm wondering how much is too much, how much is ideal to have a reasonable margin for backup power and battery longevity.




 
allen lumley
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Vlad Alba : You already have a good grasp on the basics, so I won't be much help, but lets see what other comments mine brings !

The heavier duty but basically Car batteries are supposed to last longest with only a 20 % drain down, so I am a little uncomfortable
with such a deep drain down with you very similar Golf cart batteries.

i would go with the manufacturers recommendations, possibly going to a few local Golf Courses to view the charging patterns for golf
carts. If in doubt I would take my information from the guy with a nice clean shop, safety equipment and good records, especially
ones that show increased longevity over the last several years ! Hope this help sand is timely ! Big AL
 
Chris Olson
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Location: Northern Wisconsin
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Vlad Alba wrote:Of course, my charge controller manages how high and such during the day, but I also try to use power during the day rather than the night, so depending on what I use, that cuts into the absorption/float stages. I'm wondering how much is too much, how much is ideal to have a reasonable margin for backup power and battery longevity.


If you are off-grid I would suggest that you quit guessing. You can't tell battery SOC by using voltage. And neither can you properly charge most deep cycle batteries by getting them to 28 volts for a couple hours. Full 100% SOC is reached when the electrolyte gets to 1.265-1.275 SG for flooded lead-acid batteries, so a hydrometer is the ONLY way to know if you are properly charging them. Cronic deficit charging will sulfate them and they'll be dead inside 18 months. And I have never seen a GC-2 that can be properly absorbed at 2.33 VPC.

The first thing to do is find out what your battery manufacturer recommends for Absorb V and for how long, and what they recommend for EQ V. Then charge your batteries according to their recommendation and check the cells with the hydrometer to see where they're at. Do not stop absorbing them until they get to recommended specific gravity. If you have cells that are more than 20 points difference because of sulfation and chronic deficit charging, then EQ the batteries until they "come back". Once you are at full 100% SOC you need one of these:



Properly sync'd, the TriMetric will tell you EXACTLY where your batteries are at with no guessing.

GC-2 golf cart batteries are quite fragile as far as deep cycle batteries go. They are at the absolute rock-bottom low end for off-grid systems. They can be safely discharged to 80% DoD (20% SOC) as long as they are immediately recharged to 100%. 50% DoD is the normal limit to get best efficiency and longest life. They can be cycled at 20% DoD without hurting them, however energy storage efficiency will suffer and the batteries will cost you more in the long run per kWh of storage. In all cases they must be recharged immediately, and they will last about 4-5 years.

Real off-grid or "solar" batteries can be repeatedly discharged to 80% DoD and withstand PSOC (Partial State of Charge) cycling for many days between full recharges. They typically last 10 years or longer and have low specific gravity electrolyte (1.255-1.265) with very heavy (over 1/4" thick) plates.
 
Markus Loeffler
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Location: Altadena, CA, USA
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Vlad,

The type of battery influences DOD lifetime cycles. Here is a brief description about what to expect.
Summarizing the article: with 50% DOD you get between 1-3 years of lifetime out of your batteries assuming you charge them up to 100% and discharge to 50% every day.
If that is the case I recommend you get a much bigger battery bank.

On the other hand you might not measure right. You need to buy a 'fuel gauge' type of SOC meter that is doing 'coulomb counting'.
As an alternative you can disconnect ALL batteries and let them sit for 24h and measure the voltage of each battery. That will also tell you if one of the batteries is bad.

As a rule of thumb: a battery healthy is full when the absorption rate is between 1-3% of its capacity. Lets say you have a 55AH battery then you would see the current drop to 0.5 - 1.5A at 28.8V.
 
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